The objective of this proposal is to study the function of Mullerian inhibiting substance (MIS), a member of the TGF-beta gene family, during the development of the mammalian reproductive system. During male development, the fetal testes produce MIS which actively induce the regression of the Mullerian ducts, thereby precluding Mullerian duct differentiation into a uterus, oviducts, and upper vagina. other likely male-specific functions for MIS include roles in testicular differentiation and descent, and spermatogenesis. MIS is also expressed in females in the granulosa cells of the postnatal ovary suggesting that MIS may regulate oogenesis. Our own experiments with transgenic mice chronically expressing human MIS suggest that proper MIS function is critical for the development of the male and female reproductive systems (Behringer et al., 1990). We are interested in addressing basic questions in reproductive biology that focus upon the MIS hormone. What are the potential functions of this hormone? What are its required functions during normal development and what roles can this hormone play during abnormal development? We propose in vivo experiments using transgenic mice to address these questions. We have already generated transgenic mice chronically expressing human MIS and also MIS-deficient mice by gene targeting in mouse embryonic stem (ES) cells. These MIS overexpressing and MIS- deficient mice will allow us to directly study the in vivo functions of this growth and differentiation factor during development and disease of the mammalian reproductive system.